Multi-layer seal for electrochemical devices

ABSTRACT

Multi-layer seals are provided that find advantageous use for reducing leakage of gases between adjacent components of electrochemical devices. Multi-layer seals of the invention include a gasket body defining first and second opposing surfaces and a compliant interlayer positioned adjacent each of the first and second surfaces. Also provided are methods for making and using the multi-layer seals, and electrochemical devices including said seals.

GOVERNMENT RIGHTS

[0001] This invention was made with Government support under ContractNumber DE-AC0676RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to electrochemical devices thatfunction by maintaining separate gaseous streams and causing reactionsto occur at the surfaces of conductive layers of adjacent components.

[0003] Electrochemical devices having multiple components, such as, forexample, solid oxide fuel cell (SOFC) stacks, syngas membrane reactors,oxygen generators and the like require a critical seal technology toseparate gas streams (e.g., H₂ and O₂) and to prevent the streams frommixing with each other. Mixing of the gas streams has a variety ofnegative consequences, depending upon the type of device and thecomposition of the gaseous streams. One major problem that results frommixing of such gases is the possibility of thermal combustion of thegases and the resulting failure of the device.

[0004] One type of electrochemical device that has received, andcontinues to receive, significant attention is a fuel cell device. Fuelcell devices are known and used for the direct production of electricityfrom standard fuel materials including fossil fuels, hydrogen, and thelike by converting chemical energy of a fuel directly to electricalenergy. This conversion is accomplished by oxidizing the fuel without anintermediate thermal energy stage. Fuel cells typically include a porousanode, a porous cathode, and a solid or liquid electrolyte therebetween.Fuel (e.g., hydrogen) is fed to the anode where it is oxidized andelectrons are released to the external circuit. Oxidant (e.g., oxygen)is fed to the cathode where it is reduced and electrons are acceptedfrom the external circuit. The electron flow through the externalcircuit produces direct-current electricity. The electrolyte conductsions between the two electrodes.

[0005] Fuel cells are classified into several types according to theelectrolyte used to accommodate ion transfer during operation. Examplesof electrolytes include aqueous potassium hydroxide, concentratedphosphoric acid, fused alkali carbonate, solid polymers, e.g., a solidpolymer ion exchange membrane, and solid oxides, e.g., a stabilizedzirconium oxide. Solid oxide fuel cell (“SOFC”) devices have attractedconsiderable attention as the fuel cells of the third generationfollowing phosphoric acid fuel cells and molten carbonate fuel cells ofthe first and second generations, respectively. SOFC devices have anadvantage in enhancing efficiency of generation of electricity,including waste heat management, with their operation at hightemperatures, typically above about 650° C.

[0006] Those involved in research and development of SOFC technologyconsider SOFC power generation as an emerging viable alternative to theuse of internal combustion engines. Contrary to internal combustion, theoxygen is transported in a SOFC device via the vacancy mechanism througha dense ceramic electrolyte, and then reacted with the hydrogenelectrochemically. Because the SOFC converts the chemical energy toelectrical energy without the intermediate thermal energy step, itsconversion efficiency is not subject to the Carnot Limit. Compared toconventional power generation, SOFC technology offers severaladvantages, including, for example, substantially higher efficiency,modular construction, minimal site restriction, and much lower airpollution.

[0007] In a typical SOFC, a solid electrolyte, made of denseyttria-stabllized zirconia (YSZ) ceramic, separates a porous ceramicanode from a porous ceramic cathode. The anode typically is made ofnickel/YSZ cermet, and the cathode is typically made of doped lanthanummanganite. In such a fuel cell, an example of which is shownschematically in FIG. 1, the fuel flowing to the anode reacts with oxideions to produce electrons and water. The water is removed in the fuelflow stream. The electrons flow from the anode through an externalcircuit and thence to the cathode. The oxygen reacts with the electronson the cathode surface to form oxide ions that diffuse through theelectrolyte to the anode. The electrolyte is a ceramic material that isa nonconductor of electrons, ensuring that the electrons must passthrough the external circuit to do useful work. However, the electrolytepermits the oxygen ions to pass through from the cathode to the anode.

[0008] When fuel is supplied to the anode and oxidant is supplied to thecathode, a useable electric current is electrochemically generated bythe flow of electrons through the external circuit from the anode to thecathode. As an example, the chemical reaction for a fuel cell usinghydrogen as the fuel and oxygen as the oxidant is shown in equation (1).

H₂+½O₂→H₂O  (Eq. 1)

[0009] This process occurs through two redox or separate half-reactionswhich occur at the electrodes as follows:

[0010] Anode Reaction

H₂+O²⁻→H₂O+2e⁻  (Eq. 2)

[0011] Cathode Reaction

½O₂+2e⁻→O²⁻  (Eq. 3)

[0012] In the anode half-reaction, the hydrogen fuel is oxidized byoxygen ions from the electrolyte, thereby releasing electrons (e⁻) tothe external circuit as shown in equation (2) and as shown schematicallyin FIG. 1. The oxygen ions migrate through the fuel cell electrolytefrom the cathode to the anode. In the cathode half-reaction, oxygen isfed to the cathode, where it supplies the oxygen ions (O²⁻) to theelectrolyte by accepting electrons from the external circuit. Themovement of oxygen ions through the electrolyte maintains overallelectrical charge balance, and the flow of electrons in the externalcircuit provides useful power. As alternatives to hydrogen, useful fuelsfor fuel cell power generation include, for example, carbon monoxide andmethane.

[0013] Because each individual electrochemical cell, made of a singleanode, a single electrolyte, and a single cathode, generates an opencircuit voltage of about one volt, and each cell is subject to electrodeactivation polarization losses, electrical resistance losses, and ionmobility resistant losses which reduce its output to even lower voltagesat a useful current, a fuel cell assembly comprising a plurality of fuelcell units electrically connected to each other is required to producethe desired voltage or current to generate commercially usefulquantities of power.

[0014] Currently, there are two basic designs for SOFC applications:tubular and planar. With respect to planar SOFC designs, the individualelectrochemical cells are typically connected together in series to forma stack. For example, planar solid oxide fuel cell stacks typicallycomprise a plurality of stacked cathode-electrolyte-anode-interconnectrepeat units, and the fuel cell stack includes an electricalinterconnect between the cathode and the anode of adjacent cells. Thefuel cell assembly also includes ducts or manifolding to conduct thefuel and oxidant into and out of the stack. Channels for gas flow,either in a cross-flow or a co-flow or a counterflow configuration, areusually incorporated into the cathode, anode and/or interconnect. Planardesigns are believed to potentially offer lower cost and higher powerdensity per unit volume compared to tubular designs; however, planardesigns face many challenges that must be overcome.

[0015] In addition to the challenges in materials development forelectrolytes, anodes, and cathodes, planar SOFC designs require sealsbetween each individual cell to prevent (or at least sufficientlyminimize) leaking of gases from the stack as well as mixing of fuel andoxidant gases. Low fuel leak rates are required if SOFC stacks are tooperate safely and economically. Furthermore, the seal needs to havelong-term stability at the elevated temperatures and harsh (oxidizing,reducing and humid) environments typical of SOFCs during operation.Also, the seals should not cause corrosion or other degradation of thematerials with which they are in contact (e.g., stabilized zirconia,interconnect, and electrodes). Perhaps most significantly, the sealneeds to be suitably durable to acceptably perform its sealing functionunder repetitive thermal cycling.

[0016] A variety of features of a SOFC stack add to the difficulty ofobtaining a good seal. For one, both the cell (including anode,electrolyte and cathode layers) and the interconnect, whether of ceramicor metallic material, are rigid. As a result, to achieve an effectiveseal, the mating surfaces between the cell and the interconnect must beflat and parallel. Nevertheless, because all of the components arerigid, even with good flatness, it is necessary to seal the surfaces insome manner to prevent leakage of the gases.

[0017] Another feature of electrochemical devices, such as SOFCs, thatlends to the difficulty of obtaining a good seal relates to the factthat diverse compositions are used as the components of a SOFC device,and the diverse compositions have differing thermal expansioncharacteristics. In this regard, in various types of fuel cellassemblies adapted for use at high operating temperatures, a monolithicdesign is used in which the entire structure is made of ceramics. Inother designs, individual components are rigidly and hermetically sealedusing, for example, glass seals, glass-ceramic seals, cermet seals ormetallic braze. While such monolithic or rigidly formed fuel cells arewell equipped to prevent gas leakage, ceramics have the inherentmaterial characteristic of low ductility and low toughness.Consequently, they are susceptible to damage by mechanical vibrationsand shocks. Furthermore, and perhaps more problematic, such assembliesare extremely susceptible to thermal shocks and to thermally inducedmechanical stresses due to the different thermal expansioncharacteristics of the components.

[0018] A wide variety of applications for which SOFC devices can be usedto advantage involve intermittent power demands, and thus involveintermittent usage and nonusage, and thus repeated heating and coolingcycles. Given the variety of materials used to make a single cell, andthe difficulty of selecting suitable materials that have preciselymatched coefficients of thermal expansion, it is readily seen that theuse of rigid seals presents significant problems. Furthermore, where thefuel cell is designed to be used at lower temperatures with alow-temperature ceramic electrolyte, some components of the fuel cellmay be made of metals, which are generally less expensive to fabricatethan ceramic components and have the advantage of improved ductility andfracture toughness, making them more resistant to mechanical and thermalshock damage than ceramics. However, in a fuel cell using metals for atleast some components and ceramics for at least some components, rigidsealing is perhaps an even greater problem because most alloyspotentially suitable for the SOFC interconnect application have muchhigher coefficients of thermal expansion than do ceramics, resulting inlarge thermal stresses and strains produced during operation of such afuel cell. When a metal/ceramic fuel cell is heated and cooled, thedimensions of the metal components change more than the dimensions ofthe ceramic components, leading to thermal strains within the structure.These thermal strains produce thermal stresses that can lead to failureof the ceramic components or the rigid seals between the ceramic andmetal components.

[0019] Another type of seal that has been considered for use inconnection with SOFC devices is a compressive seal. In a device designedto utilize a compressive seal, a layer of inert material is placedbetween components of the SOFC and a compressive force is applied to thecomponents and the material therebetween in an attempt to block leakagebetween the components. In comparison to rigid ceramic, glass ormetallic seals, compressive seals potentially offer several advantages.Since they are not rigidly bonded to the cells, the need for matchingcoefficients of thermal expansion (CTE) of all stack components isreduced or eliminated. The cells and interconnects are allowed to expandand contract more freely during thermal cycling and operation, therebyreducing structural degradation during thermal cycling and routineoperation. Elimination of the need for matching CTE greatly expands thelist of candidate interconnect materials, whether ceramic or metallic.The compressive seals also have two unique advantages over rigid seals.One is that cells in stacks may be reusable since they are not bondedwith one another. Secondly, it allows non-destructive post-serviceanalysis Research in the area of the compressive seals is still in itsearly stages and very little data is available. One group discussed theuse of compressed mica in a single-cell SOFC set-up; however theeffectiveness was not discussed. (Kim and Virkar, Solid Oxide Fuel Cells(SOFC VI) Proceedings of the Sixth International Symposium, edited byS.C. Sighal and M. Dokiya, The Electrochemical Society, ProceedingsVolume 99-19, 830 (1999)). A recent publication discusses work relatingto micas in paper form and cleaved single crystal micas as compressiveseals for SOFC applications. (Simner et al. “Compressive mica seals forSOFC applications,” J. Power Sources, 102 [1-2], 310-316, (2001)). Theresults showed that cleaved natural mica sheets were far superiorcompared to mica papers. For the mica sheets, leak rates of about0.33-0.65 sccm/cm at 800° C. and 100 psi were measured on small testcoupons simulating a single interconnect/seal/cell/seal/interconnectunit. A coupon leak rate of 0.33-0.65 sccm/cm, however, is believed totranslate to unacceptably high leak rates for actual SOFC stacks, inwhich multiple, full size components would be stacked together with thegaskets between each component.

[0020] In view of the above background, it is apparent that oneimportant challenge in the development of SOFC assemblies and otherelectrochemical devices is the development of sealing technologyoffering suitably low leak rates. There is a continuing need for furtherdevelopments in the field of seals for such electrochemical devices. Thepresent invention addresses this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

[0021] Accordingly, it is one object of this invention to providedevices and methods for sealing between components of an electrochemicaldevice, such as, for example, a solid oxide fuel cell stack, a syngasmembrane reactor, an oxygen generator and the like.

[0022] It is another object of this invention to provide solid oxidefuel cell stacks and other electrochemical devices that can be subjectedto wide variations in temperature without rapid failure from cracking.

[0023] It is yet another object of this invention to provide solid oxidefuel cell stacks and other electrochemical devices for which thermalexpansion match between components thereof is not required.

[0024] These and other objects of this invention are achieved by thepresent invention, which provides electrochemical devices, such as, forexample, solid oxide fuel cell devices, syngas membrane reactors, oxygengenerators and the like, that include novel multi-layer seals betweencomponents to prevent intermixing of diverse gaseous streams.

[0025] The present invention also provides solid oxide fuel cell stacksand other electrochemical devices that can be subjected to widevariations in temperature without rapid failure from cracking.

[0026] The present invention also provides solid oxide fuel cell stacksand other electrochemical devices for which thermal expansion matchbetween components thereof is not required.

[0027] The present invention also provides novel multi-layer compressiveseals that provide excellent leak barriers at high temperatures, andmethods for making and using same.

[0028] Further forms, embodiments, objects, features, and aspects of thepresent invention shall become apparent from the description containedherein.

BRIEF DESCRIPTION OF THE FIGURES

[0029] Although the characteristic features of this invention will beparticularly pointed out in the claims, the invention itself, and themanner in which it may be made and used, may be better understood byreferring to the following description taken in connection with theaccompanying figures forming a part hereof.

[0030]FIG. 1 depicts a general schematic diagram showing the function ofa solid oxide fuel cell.

[0031]FIG. 2 is a schematic diagram of an embodiment of a multi-layerseal in accordance with the invention.

[0032]FIG. 3 is a schematic diagram of a multi-layer seal as shown inFIG. 2, oriented between components of an electrochemical device.

[0033]FIG. 4 is a schematic diagram of another embodiment of amulti-layer seal in accordance with the invention, oriented betweencomponents of an electrochemical device.

[0034]FIG. 5 is a schematic diagram of an embodiment of anelectrochemical device of the invention.

[0035]FIG. 6 is a schematic diagram of another embodiment of anelectrochemical device of the invention.

[0036]FIG. 7 is an optical micrograph showing the surface texture of theas-received Phlogopite paper used in the work set forth in the Examples,showing large discrete Phlogopite flakes overlapping one another.

[0037]FIG. 8 is an optical micrograph showing the surface texture of theas-received Muscovite single crystal. The material is transparent andthe surface is very smooth and has fewer defects, although somescratches are visible.

[0038]FIG. 9 is an optical micrograph showing the surface texture of theMuscovite single crystal after 800° C. heat-treatment, as described inthe Examples, showing that, after heating, the material becomes opaqueand also develops micro-cracks.

[0039]FIG. 10 is a scanning electron micrograph showing the cleavage ofMuscovite single crystal after heat-treatment at 800° C.

[0040]FIG. 11 depicts the chemical constituents of Muscovite singlecrystal after heat-treatment at 800° C., as determined using energydispersive spectrometry.

[0041]FIG. 12 is a schematic diagram showing the experimental setup fora leak test, as described in the Examples.

[0042]FIGS. 13A and 13B are micrographs showing the surface morphologyof (A) Inconel tube which was ground with a #400 grit paper (bar=50microns) and (B) alumina substrate (bar=20 microns), which reveal thatsurface defects include continuous straight grooves (A) and irregularsintering grooves (B).

[0043]FIG. 14 is a schematic diagram showing one embodiment of amulti-layer compressive seal assembly for leak testing, as described inthe Examples.

[0044]FIG. 15 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofMuscovite mica in the single crystal form, with and without glassinterlayers, at 800° C., as described in the Examples.

[0045]FIG. 16 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofMuscovite mica paper, with and without glass interlayers, at 800° C., asdescribed in the Examples.

[0046]FIG. 17 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofPhlogopite mica paper, with and without glass interlayers, at 800° C.,as described in the Examples.

[0047]FIG. 18 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofMuscovite mica in the single crystal form, with and without metallicinterlayers, at 800° C. and a pressure gradient of 2 psi across theseal, as described in the Examples.

[0048]FIG. 19 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofMuscovite mica paper, with and without metallic interlayers, at 800° C.and a pressure gradient of 2 psi across the seal, as described in theExamples.

[0049]FIG. 20 is a plot showing the effect of the compressive stress onthe normalized leak rate of seals having a gasket body composed ofPhlogopite mica paper, with and without metallic interlayers, at 800° C.and a pressure gradient of 2 psi across the seal, as described in theExamples.

[0050]FIG. 21 is a plot showing the effect of the compressive stress onthe normalized leak rate for multi-layer seals having a gasket bodylayer of Muscovite mica in the single crystal form, and having silverinterlayers of different thicknesses, at 800° C. and a pressure gradientof 2 psi across the seal.

[0051]FIG. 22 is a schematic diagram showing the experimental setup fora leak test of multiple seals, as described in the Examples

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] For the purpose of promoting an understanding of the principlesof the invention, reference will now be made to preferred embodimentsand specific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

[0053] The present invention provides a novel manner of sealing adjacentcomponents of an electrochemical device to prevent the leakage of gasesbetween the components. In accordance with one aspect of the invention,a multi-layer seal is positioned at the junction between two adjacentcomponents of an electrochemical device and a compressive force isapplied to the components and the seal to achieve sealing. Referring tothe embodiment set forth in FIG. 2, multi-layer seal 100 is composed ofa gasket body 110 disposed between two compliant interlayers 120, 130.As used herein, the term “interlayer” refers to a layer of themulti-layer seal that, when positioned for use, lies between the gasketbody 110 and an adjacent component, as depicted in FIG. 3. Thus, when aninventive multi-layer seal is placed in a junction between adjacentcomponents, as depicted in FIG. 3, interlayer 120 is positioned betweengasket body 110 and component 140, and interlayer 130 is positionedbetween gasket body 110 and component 150. Gasket body 110 is formed todefine first and second opposing surfaces 112, 114 configured tocorrespond to surfaces 142, 152 of components 140, 150, respectively.The term “correspond” is intended to indicate that surfaces 112, 114 ofgasket body 110 are shaped to generally meet surfaces 142, 152 ofcomponents 140, 150, respectively, irrespective of whether surfaces 142,152 are planar. In an embodiment, for example, in which componentsurfaces 242, and 252 are not planar, as depicted in FIG. 4, surfaces212, 214 of gasket body 210 are formed to correspond thereto along atleast a portion of the junction. It is of course understood that theembodiment set forth in FIG. 4 is intended to provide an example of ajunction in which component surfaces 242, 252 are not planar, but is notintended to limit the invention to the surface type shown. It is wellwithin the purview of a person of ordinary skill in the art to envision,make and use alternative junction surface configurations in accordancewith the invention. It is, of course, recognized that interlayers 120,130, 220, 230 lie between the corresponding surfaces 112, 114, 212, 214of gasket body 110, 210, and surfaces 142, 152, 242, 252 of components140, 150, 240, 250, respectively. When a compressive force is applied tocomponents 140, 150, 240, 250 and multi-layer seal 100, 200, themulti-layer seal provides an effective barrier against leakage of gasesthrough the junction.

[0054] It is readily understood that in electrochemical devices, it isoften necessary to maintain discrete electrical circuits, of whichcertain components of the electrochemical device often are an integralpart. As such, it is often important to use a non-conducting orinsulating seal between the components to prevent electrical shortingwithin the device. In certain embodiments of the invention, gasket body110, 210 comprises a non-conducting material. In other embodiments, atleast one of interlayers 120, 130, 220, 230 comprises a non-conductingmaterial. In yet other embodiments, gasket body 110, 210 and interlayers120, 130, 220, 230 are formed of non-conducting materials.

[0055] In one embodiment, the gasket body comprises mica. The term“mica” encompasses a group of complex aluminosilicate minerals having alayer structure with varying chemical compositions and physicalproperties. More particularly, mica is a complex hydrous silicate ofaluminum, containing potassium, magnesium, iron, sodium, fluorine and/orlithium, and also traces of several other elements. It is stable andcompletely inert to the action of water, acids (except hydro-fluoric andconcentrated sulfuric) alkalies, convention solvents, oils and isvirtually unaffected by atmospheric action. Stoichiometrically, commonmicas can be described as follows:

AB₂₋₃(Al, Si)Si₃O₁₀(F, OH)₂

[0056] where A=K, Ca, Na, or Ba and sometimes other elements, and whereB=Al, Li, Fe, or Mg. Although there are a wide variety of micas, thefollowing six forms make up most of the common types: Biotite (K₂(Mg,Fe)₂(OH)₂(AlSi₃)₁₀)), Fuchsite (iron-rich Biotite), Lepidolite(LiKAl₂(OH, F)₂(Si₂O₅)₂), Muscovite (KAl₂(OH)₂(AlSi₃O₁₀)), Phlogopite(KMg₃Al(OH)Si₄O₁₀)) and Zinnwaldite (similar to Lepidolite, butiron-rich). Mica can be obtained commercially in either a paper form orin a single crystal form, each form of which is encompassed by variousembodiments of the invention. Mica in paper form is typically composedof mica flakes and a binder, such as, for example, an organic bindersuch as a silicone binder or an epoxy, and can be formed in variousthicknesses, often from about 50 microns up to a few millimeters. Micain single crystal form is obtained by direct cleavage from natural micadeposits, and typically is not mixed with polymers or binders.

[0057] Micas are cleavable in the direction of the basal plane, whichpermits them to split easily into optically flat films, sometimes asthin as one micron in thickness. When split into thin films, they remaintough and elastic even at high temperature. Many forms of mica aretransparent, colorless in thin sheets, resilient and generallyincompressible. With respect to electrical properties, mica has theunique combination of great dielectric strength, uniform dielectricconstant and capacitance stability, low power loss (high Q factor), highelectric resistivity and low temperature coefficient and capacitance. Itis noted for its resistance to arc and corona discharge with nopermanent injury and has little or no effect when exposed to electronicradiation dosages up to 10¹⁸ ivt. With respect to thermal properties,mica is fire proof, infusible, incombustible, and non-flammable and canresist temperatures in excess of 600° C., and significantly higher,depending upon the type of mica. It has low heat conductivity, excellentthermal stability, and may be exposed to high temperatures withoutnoticeable effect. Mica is also relatively soft and can be hand cut,machined or die-punched. It is flexible, elastic and tough, having hightensile strength, and can withstand great mechanical pressureperpendicular to plane but the lamination has cleavage and can be easilysplit into very thin leaves.

[0058] In one embodiment of the present invention, Muscovite is selectedfor use as the gasket body. In another embodiment, the gasket bodycomprises Muscovite in single crystal form. In yet another embodiment,the gasket body comprises Muscovite in paper form. In still anotherembodiment, Phlogopite is selected for use as the gasket body. Inanother embodiment, the gasket body comprises Phlogopite in singlecrystal form. In yet another embodiment, the gasket body comprisesPhlogopite in paper form. In another embodiment, the gasket bodycomprises a mica selected from the group consisting of Biotite,Fuchsite, Lepidolite, Muscovite, Phlogopite and Zinnwaldite. In anotherembodiment, the gasket body comprises synthetic mica, a variety of formsof which are available commercially. It is also contemplated that othermaterials can be selected for use in accordance with the invention asthe gasket body as would occur to a person of ordinary skill in the art.It is well within the purview of a person of ordinary skill in the artto select a mica or other material for use in forming the gasket body,depending upon the type of electrochemical device being constructed, andthe operations conditions of the device. In one embodiment of theinvention, the gasket body has a thickness of from about 25 microns toabout 2 millimeters.

[0059] As described above, the multi-layer seal also includes acompliant interlayer on each opposing surface of the gasket body. Asused herein, the term “compliant” is intended to refer to a property ofthe material whereby, under operating conditions of the electrochemicaldevice, the material has a degree of plastic deformation under a givencompressive force to block gas leakage pathways through the junction.Such gas leakage pathways can result, for example, from defects in theadjacent surfaces of the components, or other irregularities in thesurfaces, including grooves on a metal component or grooves or voids ona ceramic component. Materials that can be used to form the compliantinterlayers in various embodiments include, for example and withoutlimitation, a glass, a glass-ceramic, a mica glass-ceramic, aphase-separated glass, a glass composite, a cermet, a metal, a metalalloy and a metal composite. To make a multi-layer seal in accordancewith the invention, compliant interlayers can be applied to the gasketbody in a variety of manners, including, for example and withoutlimitation, dip-coating, painting, screen printing, deposition,spattering, tape casting and sedimentation. In addition, the compliantinterlayer material can be provided in a variety of forms, including,for example, as fibers, granules, powders, slurries, liquid suspensions,pastes, ceramic tapes, metallic foils and others.

[0060] As stated above, compliant interlayers deform under a compressiveforce and under operating conditions of an electrochemical device toconform to the surfaces, including surface irregularities, of adjacentcomponents, and to thereby provide a barrier to leakage of gases underoperating temperatures and conditions of the electrochemical device. Animportant feature of the invention is the ability of at least one of thematerial interfaces at the junction, such as, for example, acomponent/interlayer interface or an interlayer/gasket body interface,to remain unbonded during operation of the electrochemical device,including heating and/or cooling cycles. As used herein, the term“unbonded” with respect to such materials at an interface is intended tomean that the components in contact at the interface are free to move(i.e., expand, contract or slide) independently of one another. Thus,during heating and/or cooling of the electrochemical device, the firstcomponent, first interlayer, gasket body, second interlayer and secondcomponent do not form a rigid mass, as occurs, for example, in deviceshaving glass seals, which melt under operating temperatures and thenrigidly bond adjacent parts to one another upon cooling.

[0061] In one embodiment of the invention, an interlayer of themulti-layer seal comprises a compliant material, such as, for example, ametallic material, that has a melting temperature greater than theoperating temperature of an electrochemical device in which themulti-layer seal is to be used. In this embodiment, the interlayer doesnot melt, and does not become bonded to a component or the gasket bodyunder operating conditions. In another embodiment, the gasket bodycomprises mica, which has been found by the present inventors to besatisfactorily resistant to bonding with materials it contacts duringoperation at high temperatures whether or not the interlayer comprises amateriel that melts under the operating temperatures. In anotherembodiment, the mica is a single crystal mica. While it is not intendedthat the present invention be limited by any theory whereby it achievesits advantageous result, it is believed that an interlayer that meltsunder operating temperatures may bond to the surface of a single crystalmica upon cooling, but that the physical properties of the mica allows afew sublayers of the mica crystal to cleave from the gasket body, thusmaintaining a non-bonded interface.

[0062] In one embodiment, the compliant interlayer comprises glass.Although a wide variety of glass compositions can be used, as wouldoccur to a person of ordinary skill in the art, it is important that theglass composition selected for use have a softening point lower than orequal to the operating temperature of the device in which the seal is tobe used. Softening point, or softening temperature, of glass is definedunder ASTM C338 as the temperature at which a uniform fiber of glass(0.55-0.75 mm diameter and 23.5 cm length) elongates under its ownweight at 1 mm/min when the upper 10 cm is heated at 5° C./min. Forglass having a density of 2.50 g/cc, this corresponds to a viscosity ofabout 10^(6.6) Pa.s. It is also desirable when selecting a glasscomposition for the interlayer to select a glass composition that is notcorrosive to surfaces of the components that come into contact with theinterlayer under operating conditions, such as, for example, metalliccomponents, ceramic components, and the gasket body. In one embodiment,the glass composition selected for use is an aluminosilicate glass. Inanother embodiment, the glass composition selected for use is aborosilicate glass. In another embodiment, the glass selected for useincludes an alkaline earth element, such as, for example, strontium,magnesium and/or calcium, or an alkali additive, such as, for example,sodium, potassium and/or lithium. In other embodiments the interlayercomprises a glass-ceramic material (i.e., a glass-ceramic material asdescribed in Lahl et al., “Aluminosilicate glass ceramics as sealant inSOFC stacks,” in Solid Oxide Fuel Cells (SOFC VI) Proceedings of theSixth International Symposium, edited by S.C. Singhal and M. Dokiya, TheElectrochemical Society, Proceedings Volume 99-19, 1057-1065 (1999)) ora mica glass-ceramic material (i.e., a mica glass-ceramic material asdescribe in Yamamoto et al, “Compatibility of mica glass-ceramics asgas-sealing materials for SOFC,” Denki Kagaku 64 [6] 575-581 (1996)).

[0063] In another embodiment of the invention, the multi-layer sealcomprises at least one metallic interlayer. In one embodiment, themetallic interlayer comprises a noble metal, such as, for example, gold,silver, palladium, or platinum. In another embodiment, the metallicinterlayer comprises a high-temperature alloy. It is also contemplatedthat other metals can be used that are resistant to oxidation underoperating conditions of the electrochemical device. Metallic interlayerscan be conveniently provided in the form of a metallic foil, such as afoil having a thickness of from about 0.005 mm to about 1 mm. In anotherembodiment, the metallic foil has a thickness of from about 0.01 mm toabout 0.5 mm. In one embodiment of the invention, the interlayercomprises silver. In another embodiment, the interlayer comprises asilver foil having a thickness of from about 1 mil (25 microns) to about10 mil (250 microns). In still another embodiment, the interlayercomprises a silver foil having a thickness of about 5 mils. Metalliclayers comprising other metals in various embodiments can also beprovided in the form of foils, including foils having thicknesses as setfor above.

[0064] To seal a junction between adjacent components of anelectrochemical device, a multi-layer seal as provided herein ispositioned between the adjacent components such that each compliantinterlayer is positioned between the gasket body and one of thecomponents. Sealing is then accomplished by applying a compressive forceto the components and the seal to maintain the seal in position and tocause the compliant interlayers to mold to surface defects in thesurfaces of the components and the gasket body under operatingconditions of the device. In one embodiment of the invention, thecompressive force is a force of from about 5 to about 500 pounds persquare inch (psi). In another embodiment, the compressive force is aforce of from about 10 to about 400 psi. In another embodiment, thecompressive force is a force of from about 15 to about 300 psi.

[0065] In operation of an electrochemical device including an inventiveseal, the temperature of the seal increases as the temperature of theelectrochemical device increases toward its normal operatingtemperature. In an embodiment comprising a glass interlayer, as thetemperature increases beyond the softening temperature of the glass, theglass, under the compressive force, deforms to mold to surfaceirregularities. When a metallic interlayer is used, the same phenomenonoccurs if the operating temperature of the electrochemical deviceexceeds the melting point of the metallic material selected for use.Even if the operating temperature does not exceed the melting point ofthe metallic material, the metallic interlayer selected for use inaccordance with the invention deforms under the compressive force toeffectively form a barrier to leakage of gases through the junctionduring operation of the device.

[0066] It is readily understood that a variety of electrochemicaldevices, such as, for example and without limitation, solid oxide fuelcells, syngas membrane reactors, and oxygen generators, have a pluralityof adjacent functional units to increase the efficiency of the deviceand to increase output of the device to a more useful level, whether theoutput of the device is electricity, synthetic gas, oxygen or other.Arrangement of a plurality of units is often accomplished by providingstacked planar units. It is readily understood that such planar unitsact as boundaries between diverse gaseous streams, and, when in astacked arrangement, form a plurality of junctions therebetween.

[0067] In one aspect of the invention, therefore, an electrochemicaldevice having a plurality of adjacent components is provided thatincludes an inventive multi-layer seal positioned at one or morejunction, preferably at multiple junctions. In a preferred embodiment,the device includes an inventive multi-layer seal at each such junction.Referring now to FIG. 5, electrochemical device 300 includes multiplecomponents, for example, components 340, 350, 360, 370, 380, 390 withmulti-layer seals positioned therebetween, for example, seals 345, 355,365, 375, 385, 395. Device 300 also includes a compression member 301configured to exert a compressive force to the components and the seal.In one embodiment, as depicted in FIG. 6, compression member 301includes end plates 302, 303 and device 300 is compressed and maintainedin its assembled state between the end plates 302, 303. In oneembodiment, compression member 301 includes one or more metal tie rodsor tension members. The tie rods can extend through holes formed in endplates 302, 303, and have associated nuts or other fastening means tosecure them in the stack assembly. The tie rods can be external, thatis, not extending through the fuel cell units, or one or more internaltie rods can be used which extend between the stack end plates throughopenings in the fuel cell units as, for example, described in U.S. Pat.No. 5,484,666. In other embodiments, compression member 301 includessprings, hydraulic or pneumatic pistons, pressure pads or otherresilient compressive means to cooperate with tie rods and end plates,or as alternatives to tie rods, to urge the two end plates towards eachother to compress the fuel cell stack components. It is not intendedthat the present invention be limited by the type of compression memberselected for application of the compressive force, and a variety ofsuitable mechanisms are well within the purview of a person of ordinaryskill in the art. Further examples are provided in U.S. Pat. Nos.4,478,917 and 5,176,966, which are hereby incorporated by referenceherein in their entireties.

[0068] In one embodiment of the invention, the electrochemical device isa solid oxide fuel cell (“SOFC”) assembly for electrochemically reactinga fuel gas with a flowing oxidant gas at an elevated temperature toproduce a DC output voltage. The SOFC includes a plurality of generallyplanar integral fuel cell units. Referring, for example to FIG. 5, in aSOFC stack assembly, each of components 340, 350, 360, 370, 380, 390 isan integral fuel cell unit comprising a layer of ceramic ion conductingelectrolyte disposed between a conductive anode layer and a conductivecathode layer. The units are arranged one on another along alongitudinal axis (Y) perpendicular to the planar units to form a fuelcell stack. Multi-layer seals 345, 355, 365, 375, 385, 395 are disposedbetween the anode layer and the cathode layer of adjacent fuel cellunits, optionally with an interconnect (not shown) also positioned incontact with the anode and/or cathode layer. The SOFC also includes acompression member 301 configured to exert a compressive force along thelongitudinal axis.

[0069] As will be appreciated by a person of ordinary skill in the artin view of the present description, in one form of the presentinvention, a multi-layer seal for sealing a junction between adjacentcomponents of an electrochemical device is provided. The multi-layerseal includes a gasket body defining first and second opposing surfaces;a first compliant interlayer positioned adjacent the first surface; anda second compliant interlayer positioned adjacent the second surface. Inone embodiment, the opposing surfaces of said gasket body are configuredto correspond to junction surfaces of the adjacent components of theelectrochemical device. In another embodiment, each of said first andsecond compliant interlayers is positioned to be disposed between thegasket body and the junction surface of one of the adjacent components.In yet another embodiment, the gasket body comprises a single crystalmica or a mica paper. The mica can be, for example, Muscovite,Phlogopite, Biotite, Fuchsite, Lepidolite or Zinnwaldite. In stillanother embodiment, the at least one of the compliant interlayerscomprises a member selected from the group consisting of a glass, aglass-ceramic, a mica glass-ceramic, a phase-separated glass, a glasscomposite, a cermet, a metal, a metal alloy and a metal composite.

[0070] In one preferred embodiment, at least one of the compliantinterlayers comprises glass. In another embodiment, the glass has asoftening point lower than or equal to the operating temperature of theelectrochemical device. In another preferred embodiment, at least one ofthe compliant interlayers comprises a metal. In another embodiment, themetal is selected from the group consisting of gold, silver, palladiumand platinum. In still another embodiment, at least one of the compliantinterlayers is a metallic foil having a thickness of from about 0.005millimeters to about 1 millimeter. In yet another embodiment, at leastone of the compliant interlayers is a metallic foil having a thicknessof from about 0.01 millimeters to about 0.5 millimeters.

[0071] In another form of the invention, an electrochemical device isprovided that includes a plurality of components, the components formingat least one boundary between diverse gaseous streams and defining atleast one junction between the components. A multi-layer seal ispositioned at the junction, the seal composed of a gasket body disposedbetween two compliant interlayers. With the seal thus positioned, eachcompliant interlayer is positioned between the gasket body and one ofsaid components. The electrochemical device also includes a compressionmember for exerting a compressive force to the components and the seal.

[0072] In certain embodiments, the seal is a non-conducting seal. In oneembodiment, the gasket body comprises mica. In another embodiment, thegasket body of the multi-layer seal has a thickness of from about 25microns to about 2 millimeters. In yet another embodiment, at least oneof the compliant interlayers comprises glass. In certain embodiments,the glass has a softening point lower than or equal to the operatingtemperatures of the device. The glass is preferably a glass that is notcorrosive to surfaces of the components in contact with the glass underoperating conditions. In one embodiment, the glass composition selectedfor use is an aluminosilicate glass. In another embodiment, the glasscomposition selected for use is a borosilicate glass. In anotherembodiment, the glass selected for use includes an alkaline earthelement, such as, for example, strontium, magnesium and/or calcium, oran alkali additive, such as, for example, sodium, potassium and/orlithium. In yet another embodiment, each of the glass interlayers has athickness of from about 0.005 millimeters to about 5 millimeters priorto heating. In still another embodiment, the glass interlayer has athickness of from about 0.05 millimeters to about 0.5 millimeters priorto heating.

[0073] In another embodiment of the electrochemical device, at least oneof the compliant layers comprises a metal. The metal selected for use ispreferably resistant to oxidation under operating conditions of thedevice. In one embodiment, the metal is selected from the groupconsisting of gold, silver, palladium and platinum. In anotherembodiment, the compliant interlayer is a metallic foil having athickness of from about 0.005 millimeters to about 1 millimeter prior toheating.

[0074] Another form of the invention is a method for making amulti-layer seal, comprising (1) providing a gasket body defining firstand second generally flat opposing surfaces; and (2) applying acompliant material to said first and second surfaces to form first andsecond compliant interlayers. In certain embodiments, the gasket bodycomprises mica and/or at least one of the compliant interlayerscomprises a member selected from the group consisting of a glass, aglass-ceramic, a mica glass ceramic, a phase-separated glass, a glasscomposite, a cermet, a metal, a metal alloy and a metal composite. Thecompliant material can be applied to the first and second surfaces ofthe gasket body, for example, by dip-coating, painting, screen printing,deposition, spattering, tape casting or sedimentation.

[0075] In another form of the invention, there is provided a method forsealing a junction between adjacent ceramic or metallic components of anelectrochemical device, comprising (1) positioning between the adjacentcomponents a multi-layer seal composed of a gasket body disposed betweena first compliant interlayer and a second compliant interlayer, whereineach of the first and second compliant interlayers is positioned betweenthe gasket body and one of the components; and (2) applying acompressive force to the components and the seal. In one embodiment, thecompressive force is a force of from about 5 to about 500 psi.

[0076] In another form of the invention, there is provided a solid oxidefuel cell assembly for electrochemically reacting a fuel gas with aflowing oxidant gas at an elevated temperature to produce a DC outputvoltage, said solid oxide fuel cell. The SOFC assembly includes aplurality of generally planar integral fuel cell units, each unitcomprising a layer of ceramic ion conducting electrolyte disposedbetween a conductive anode layer and a conductive cathode layer, and theunits are arranged one on another along a longitudinal axisperpendicular to the planar units to form a fuel cell stack. Theassembly also includes a multi-layer non-conducting seal disposedbetween the anode layer and the cathode layer of adjacent fuel cellunits. The seal is composed of a gasket body disposed between twocompliant interlayers. The assembly also includes a compression memberfor exerting a compressive force along the longitudinal axis. In oneembodiment, the compressive force is a force of from about 5 to about500 psi.

[0077] Reference will now be made to specific examples illustratingvarious preferred embodiments of the invention as described above. It isto be understood that the examples are provided to illustrate preferredembodiments and that no limitation to the scope of the invention isintended thereby.

EXAMPLE ONE Leak Test Comparitive Data

[0078] 1. Plain Mica Seal (Compressive Mica Seals)

[0079] Three micas were used in this study: Muscovite(KAl₂(AlSi₃O₁₀)(F,OH)₂) paper, cleaved Muscovite single crystal sheet,and Phlogopite (KMg₃(AlSi₃O₁₀)(OH)₂) paper. All three micas are about0.1 mm thick. The Muscovite mica loses about 4% chemical water at about600° C. Phlogopite mica is more stable in high temperatures, losing itschemical water at about 950° C. The paper-type micas are composed ofdiscrete large mica flakes bonded with organic binders, and pressed intothin sheets. FIG. 7 shows the typical surface morphology of thePhlogopite mica paper. It is clear that there are large voids(un-overlapped regions) and the surface is relatively rough. TheMuscovite mica paper also shows similar surface features. On the otherhand, the cleaved Muscovite single crystal mica sheets are transparentas received and have much smoother surfaces, though there are somescratches present (FIG. 8). After treatment at 800° C., the singlecrystal lost chemical water, which resulted in more defects of thesurface (FIG. 9). It also cleaved into many parallel sub-layers (FIG.10). Energy dispersive spectrometry (EDS, FIG. 11) showed the Muscovitesingle crystal also contained small amounts of Na, Fe and Ti in additionto the major constituents (K, Al and Si).

[0080] Mica samples 410 were cut into 1½ inch squares with a ½ inchdiameter central hole. The mica squares 410 were then pressed between anInconel tube 420 (outer diameter=1.3 inch and inner diameter=1.0 inch)and a dense alumina substrate 430. Samples were heated in a clamshellfurnace 440 at a heating rate about 2° C./min to 800° C. The load wasapplied using a universal mechanical tester 450 with a constant loadcontrol (Model 5581, Instron, Canton, Mass.). The experimental setup isshown schematically as in FIG. 12. A large known-volume (370 cm³)reservoir 460 was kept at ambient conditions and connected to the samplevia a ⅛ inch Cu tube 462. By setting up a vacuum in the system viavacuum pump 464 (initially as low as about 100 mtorr), the leak rate wasmeasured using pressure sensor 466 by monitoring the pressure changewith time. The final pressure was about 2 torr. The pressure gradientacross the mica seal therefore could be considered to be essentiallyconstant at 14.7 psi. Assuming the ideal gas law, the leak rate (L) wascalculated by the equation:

L=Δn/Δt=(n _(f) −n _(i))/(t _(f) −t _(i))=(p _(f) −p _(i))V/RT(t _(f) −t_(i))

[0081] where n is the moles of the gas, T is the temperature, V is thereservoir volume, R is the gas constant, t is the time, and p is thepressure. Subscripts f and i represent the final and the initialconditions. The calculated leak rate (L, in standard cubic centimetersper minute at STP, sccm) was further normalized with respect to theouter leak length (10.5 cm) of the Inconel tube and to a pressuregradient of 2 psi by the equation:

{overscore (L)}=L×2/(10.5×14.7)

[0082] Thus, the unit for leak rate is standard cubic centimeters perminute per centimeter (“sccm/cm”) at STP and with a pressure gradient of2 psi.

[0083] Before each run, the leak rate of the background (or the systemwithout test samples) was also measured and subtracted from the actualtest runs. To ensure a constant temperature, all leak tests wereconducted about ½ hour after reaching the desired temperatures (800°C.).

[0084] Data collected for the plain mica seal comparative test runs isset forth in Table I below. TABLE I Normalized leak rate (sccm/cm) forplain compressive mica seals at 800° C. Compressive Muscovite MuscovitePhlogopite stress (psi) single crystal paper paper 100 0.66 5.77 8.85300 0.42 2.84 2.97 500 0.28 1.92 1.68

[0085] 2. Rigid GlassSeal

[0086] For comparison, a test of a glass-only seal was also conductedusing the same setup as described above and using a single layer ofglass without the mica sheet. It is worthy of note that, irrespective ofthe form of the glass prior to the test, the glass softened at thetemperatures used in the test, and then resolidified as the apparatuscooled after testing to form a rigid glass seal. For a rigid glass seal,it was found that the leak rate was about 5×10⁻⁵ sccm/cm at 800° C. at apressure gradient of 2 psi, using the current test setup. Ideally, theleak rate should be zero for a hermetic seal if the glass wets thesurfaces in contact. In reality, the actual low leak rates were limitedby the system's background since there were valves and tube connectorsin the setup.

[0087] 3. Discussion of Comparative Data

[0088] As for the compressive mica seals, Simner et al. reported a leakrate of 0.65 sccm/cm at the same conditions and a compressive stress of100 psi using the Muscovite single crystal sheet (about 0.1 mm thick).(S. P. Simner and J. W. Stevenson, “Compressive mica seals for SOFCapplications,” J. Power Sources, 102 [1-2] 310-316 (2001)). It isappropriate to ask why the apparently flexible thin mica sheet allowed aleak rate about 10⁴ times higher than that of a glass seal (theas-received Muscovite single crystal sheet, though relatively stiff inits as-received form, becomes flexible (and fragile) when heated to 800°C.). Looking at a compressed mica between the Inconel tube and thealumina substrate, one can imagine there are two possible paths forleaks. One is from the interface between the metal tube (or the ceramicsubstrate) and the mica. The other one is through the mica itself, sinceit cleaves into many sub-layers after losing its chemically bonded waterat elevated temperatures (Simner et al.). Looking at the surfaces of thecontact materials (dense alumina as the support substrate and theInconel tube as the top pressing ram, FIGS. 13A and 13B), it was evidentthat many defects were present, including long grooves on the metal andthe irregular grooves (voids) on the ceramic substrate. Therefore, itseemed likely that the major leaks for the compressive mica sealoccurred through these interfaces.

EXAMPLE TWO Leak Test Mica Gasket Body with Glass Interlayers

[0089] The same protocol was used as in Example 1, but the mica sealswere replaced with multi-layer seals, in which glass interlayers 412,414 were placed between the Inconel tube 420/mica 416 and mica416/alumina 430 interfaces as shown in FIG. 14. For the glassinterlayers, a borosilicate glass filter paper was used. Theborosilicate glass contains about 58% SiO₂, about 9%B₂O₃, about 11%Na₂O,about 6%Al₂O₃, about 4%BaO, and ZnO, CaO and K₂O. The results seal areset forth in Table II below. TABLE II Normalized leak rate (sccm/cm) forthe multi-layer seals (glass interlayers) at 800° C. CompressiveMuscovite stress (psi) single crystal Muscovite paper Phlogopite paper25 0.000359 — — 50 0.000243 — — 100 0.000155 0.0126 0.0108 200 — 0.01220.0105 300 — 0.0115 0.0103 400 — 0.0107 0.0098 500 — — —

[0090] Ir is seen that the best results were obtained using Muscovitesingle crystal mica. The normalized leak rate for this seal at 800° C.was only 1.55×10⁻⁴ sccm/cm at a stress of 100 psi and a pressuregradient of 2 psi, which is a leak rate about 4300 times lower than theleak rate of a simple mica seal at this temperature. Seals based on theother commercial micas (Muscovite and Phlogopite mica papers), alsoexhibited superior leak rates (about 0.011 sccm) compared to simple micaseals without the compliant glass interlayer (about 6 to about 9sccm/cm).

[0091] The multi-layer seals with glass interlayers are shown to exhibitexcellent sealing function for electrochemical devices, considering thelow leak rates reported above. For a 60-cell (14 cm×14 cm active areaper cell) stack, producing 0.5 W/cm² or 5.9 kW total gross power onsteam reformed methane (steam to carbon mole ratio of 3.0), at 65% fuelutilization, 20% oxygen utilization, the total reformate gas flow rateentering the anode is estimated to be 1.36×10⁵ sccm (STP). Assuming thatthe leak rate (per cm of seal length) measured in this study applied tofull size stacks, the total leak rate for a 60-cell stack at 800° C.would be only 0.0019% of the total fuel rate for the multi-layer sealincluding a Muscovite single crystal mica gasket body and glassinterlayers under a stress of 25 psi and a 2 psi pressure gradient (aleak length of 124 cm was assumed for each layer).

EXAMPLE THREE Materials Damage Analysis Mica Gasket Body with GlassInterlayers

[0092] The microstructure of the mica was examined before and after the800° C. leak tests using scanning electron microscopy to assess whetherthe use of a low melting glass as the seal interlayer could damage thematerials with which it is in contact (e.g., metal, ceramic, and themica itself), especially under the compressive stresses. Thoughlong-term stability tests are underway, preliminary results showed nosubstantial corrosion or melting of the materials in contact. Thecorrosion at the glass metal/interface was limited to a depth of a fewmicrons. This may result from the fact that the majority of the glasswas squeezed out from between the components at elevated temperaturesunder the compressive stresses. If only a thin glass interlayer is leftbehind, only limited corrosion or melting would be likely to occur. Asfor the mica itself, degradation might be expected due to interactionbetween the mica (an aluminosilicate) and the borosilicate glass, but nosignificant degradation was observed. The mica remained intact exceptfor a few surface sub-layers which bonded to the metal tube and theceramic substrate when the test specimens were disassembled aftertesting.

EXAMPLE FOUR Effects of Compressive Stress on Leak Rate Mica Gasket Bodywith Glass Interlayers and Comparitive Data

[0093] The 800° C. leak rates for the three micas, with and withoutglass interlayers and at various compressive stesses are summarized inTable III below. TABLE III Normalized leak rate (sccm/cm) for themulti-layer seals and plain compressive mica seals at 800° C. MuscoviteCompressive single crystal Muscovite paper Phlogopite paper stress (psi)plain multi-layer plain multi-layer plain multi-layer 25 — 0.000359 — —— — 50 — 0.000243 — — — — 100 0.66 0.000155 5.77 0.0126 8.85 0.0108 200— — — 0.0122 — 0.0105 300 0.42 — 2.84 0.0115 2.97 0.0103 400 — — —0.0107 — 0.0098 500 0.28 — 1.92 — 1.68 —

[0094] The results are also plotted as a function of the compressivestresses for Muscovite single crystal mica sheet (FIG. 15), Muscovitemica paper (FIG. 16), and Phlogopite mica paper (FIG. 17). It is evidentthat the leak rates were greatly reduced for multi-layer seals ascompared to the plain compressive mica seals. For example, the Muscovitesingle crystal mica showed an extremely low leak rate of 1.55×10⁻⁴sccm/cm 800° C. and a compressive stress of 100 psi. As for the plainMuscovite single crystal mica, i.e. without the glass interlayers, theleak rates at the same test conditions were 0.66 sccm/cm, approximately4300 times higher. Similar behaviors were observed for the paper type(discrete mica flakes bonded with organic binders) Muscovite andPhlogopite micas. For example, the leak rates for the multi-layer sealincluding a Muscovite paper gasket body and glass interlayers were0.0126 sccm/cm, about 460 times lower than the plain Muscovite micapaper (5.77 sccm/cm) at 800° C. and a stress of 100 psi. The leak ratesfor the multi-layer seal including a Phlogopite mica paper gasket bodyand glass interlayers were 0.0108 sccm/cm, about 820 times lower thanthat of the plain Phlogopite mica paper (8.85 sccm/cm). The resultsclearly indicate that the major leaks occurred at the Inconel tube/micaand mica/ceramic substrate interfaces.

[0095] It is also interesting to note that the effect of increasing theapplied compressive stress was much weaker for the multi-layer sealsthan for the plain mica seals. This is especially clear for thepaper-type micas. For example, the leak rate reduced about 81% (from8.85 sccm/cm to 1.68 sccm/cm) for Phlogopite mica paper when thecompressive stress increased 400% from 100 psi to 500 psi. For themulti-layer form, the leak rate only reduced about 10% (from 0.0108sccm/cm to 0.0098 sccm/cm) for a 300% increase in the stress from 100psi to 400 psi. Similar results were also evident for the Muscovite micapaper. No substantial difference was observed between the Phlogopitemica paper and the Muscovite mica paper, though the former is morestable at higher temperatures than the latter. These results areconsistent with previously reported data. Simner et al. reported asimilar reduction for a thicker Phlogopite paper (0.5 mm) while usinghigh purity helium at a 2 psi positive pressure gradient; in that studythe leak rate dropped about 85% from 6.26 sccm/cm to 0.97 sccm/cm whenthe applied stress increased from 100 psi to 500 psi. (Simner et al.).

[0096] In the case of the Muscovite single crystal mica, there was astrong dependence on the applied compressive stress for both themulti-layer seals and the plain mica seals. However, the stress rangefor the multi-layer seal with single crystal mica was only from 25 psito 100 psi (at higher stresses the leak rates were close to the system'sbackground, so tests were not conducted). It is expected that themulti-layer seal with single crystal mica would also show lessdependence on stress at higher compressive loads since the sub-layers(after the loss of chemical water at elevated temperatures FIG. 10) aremore closely overlapped with each other. The fact that the multi-layerseals were less dependent on the compressive stress is consistent withthe fact that the major leaks occur at the interfaces between the micaand the metal tube or the interface between the mica and the ceramicsubstrate. The glass used as the interlayer is a borosilicate glasswhich melts at around 600° C. and therefore can fill and seal surfacedefects such as long grooves or voids.

[0097] Overall, it is clear that the Muscovite single crystal micasoffer superior performance to the mica papers in multi-layer seals; forexample, the leak rate for a Muscovite multi-layer seal was only3.59×10⁻⁴ sccm/cm at a low compressive stress of 25 psi. Based uponmicrostructural examination of these materials, this is likely due tothe fact that the paper type micas are composed of discrete micaflakes/platelets, so that the leak paths are 3-dimensional, whereas thesingle crystal micas tend to have only 2-dimensional leak paths (throughthe cleavage planes). Though the single crystal mica sheets did formsome defects after the loss of chemical water at elevated temperatures,these defects (micro-cracks) were minute in size compared to theconnected voids which were prevalent in the mica papers.

EXAMPLE FIVE Leak Test Mica Gasket Body with Metallic Interlayers

[0098] The protocol set forth in Example Two was repeated using amulti-layer seal composed of a mica gasket body and two compliantmetallic interlayers. The metallic material used as the compliant layersin these tests was silver in the form of a thin foil.

[0099] The results of testing of multi-layer seals having metallicinterlayers are set forth in Table IV below, which also includes datafor the plain compressive mica seals and multi-layer seals with glassinterlayers, as set forth in Table III. TABLE IV Normalized leak rates(sccm/cm) for the plain mica seals and the multi-layer seals at 800° C.Muscovite single crystal Muscovite paper Phlogopite paper Compressivemulti-layer multi-layer multi-layer multi-layer multi-layer stress (psi)plain (glass) (1 mil Ag) (5 mil Ag) plain (1 mil Ag) plain (1 mil Ag) 25— 0.000359 0.0047 0.0019 — 0.134 — 0.164 50 — 0.000243 0.0040 0.0014 —0.114 — 0.134 100 0.66 0.000155 0.0030 0.00089 5.77 0.094 8.85 0.098 200— — — 0.00036 — — — — 300 0.42 — 0.0012 0.00022 2.84 0.060 2.97 0.053400 — — — — — — — — 500 0.28 — 0.00043 — 1.92 0.044 1.68 0.037

[0100] It is seen that the leak rates are also greatly reduced for themulti-layer seals with metallic interlayers as compared to plaincompressive mica seals. It is also seen that the best results were againobtained using Muscovite single crystal mica. The normalized leak ratefor the multi-layer seal with 5 mil silver interlayers at 800° C. wasonly 8.9×10⁻⁴ sccm/cm at a stress of 100 psi and a pressure gradient of2 psi, which is a leak rate about 740 times lower than the leak rate ofa simple mica seal at this temperature. Seals based on the othercommercial micas (Muscovite and Phlogopite mica papers), also exhibitedsuperior leak rates (about 9.5×10⁻² sccm/cm) compared to simple micaseals without the compliant metallic interlayer (about 6 to about 9sccm/cm).

[0101] The multi-layer seals with metallic interlayers are also shown toexhibit excellent sealing function for electrochemical devices,considering the low leak rates reported above. For a 60-cell (14 cm×14cm active area per cell) stack, producing 0.5 W/cm² or 5.9 kW totalgross power on steam reformed methane (steam to carbon mole ratio of3.0), at 65% fuel utilization, 20% oxygen utilization, the totalreformate gas flow rate entering the anode is estimated to be 1.36×10⁵sccm (STP). Assuming that the leak rate (per cm of seal length) measuredin this study applied to full size stacks, the total leak rate for a60-cell stack at 800° C. would be only 0.026% of the total fuel rate forthe multi-layer seal including a Muscovite single crystal mica gasketbody and metallic interlayers under a stress of 25 psi and a 2 psipressure gradient (a leak length of 124 cm was assumed for each layer).

EXAMPLE SIX Effects of Compressive Stress on Leak Rate Mica Gasket Bodywith Metallic Interlayers and Comparitive Data

[0102] Leak rates were also determined for the three micas at variouscompressive pressures, and the results are summarized in Table IV above.Table IV includes data of the multi-layer seals (with glass interlayersand metallic interlayers) and the plain compressive mica seals. Theresults are also plotted as a function of the compressive stresses forMuscovite single crystal mica sheet (FIG. 18), Muscovite mica paper(FIG. 19), and Phlogopite mica paper (FIG. 20). It is evident that theleak rates were greatly reduced for the multi-layer seals with metallicinterlayers as compared to the plain compressive mica seals.

[0103] The effect of increasing the applied compressive stress was alsomuch weaker for the multi-layer seals with metallic interlayers than forthe plain mica seals. This is especially clear for the paper-type micas.For example, the leak rate reduced about 81% (from 8.85 sccm/cm to 1.68sccm/cm) for Phlogopite mica paper when the compressive stress increased400% from 100 psi to 500 psi (Table 1). For the multi-layer form, theleak rate only reduced about 62% (from 9.8×10⁻² sccm/cm to 3.7×10⁻²sccm/cm) for the same increase in the stress. Similar results were alsoevident for the Muscovite mica paper. No substantial difference wasobserved between the Phlogopite mica paper and the Muscovite mica paper,though the former is more stable at higher temperatures than the latter.These results are also consistent with previously reported data.

EXAMPLE SEVEN Effect of Metallic Interlayer Thickness on Leak Rate

[0104]FIG. 21 shows the 800° C. leak rates of the multi-layer seal usingMuscovite single crystal mica and silver interlayers of two thicknesses:1 mil (25 microns) and 5 mil (125 microns). It is evident that thethicker silver interlayers have lower leak rates. For example, the leakrates at a compressive stress of 100 psi were 8.9×10⁻⁴ sccm/cm using the5 mil Ag layers, and were 3.0×10⁻³ sccm/cm using the 1 mil Ag.

[0105] The thickness variation along the pressed ring (1.3 inch outsidediameter, and 1.0 inch inner diameter) sections was measured with adigital micrometer along the 12 hour positions. Alumina substrate wasfound to be more uniform in the thickness (with a maximum thickness of514 microns and a minimum thickness of 507 microns), whereas theas-received Muscovite single crystal mica varies more in the thickness(with a maximum thickness of 122 microns and a minimum thickness of 107microns). In view of the 7 micron fluctuation on the Alumina substratesurface and the 15 micron fluctuation in the mica surface, theworst-case gap between the surfaces would be 7+15=22 microns. Adetermination of surface variation was not made for the Inconel tubesurface.

[0106] The difference in FIG. 21 is likely due to the thicknessvariation of samples used that are not perfectly flat (i.e., not thesame thickness throughout the pressed ring section). Using a thickersilver foil appears to be more effective in space/defect filling for thecomponents used in this test. It is also worthy of note, however, thatthinner (1 mil) silver interlayers provide adequate seals for mostapplications, and are less expensive.

EXAMPLE EIGHT Glass Interlayer Test with Multiple Stacked ComponentsHaving Multi-Layer Seals Therebetween

[0107] A simulated multiple component assembly was also tested. In thistest, six multi-layer seals having gasket bodies of Muscovite singlecrystal mica and borosilicate glass interlayers were used. Five layersof metal sheet (SS 430 with a nominal sheet thickness of 0.010 inches)were placed between the multi-layer seals, as depicted in FIG. 22. An8-cycle run was performed in which the temperature of the device wascycled between 100° C. and 800° C. under a compressive stress of 100psi. The results of this leak test are set forth below in Table V. Foreach cycle multiple measurements were conducted as shown in Table V.TABLE V Total and normalized leak rates of an assembly having sixmulti-layer seals after cycling from 100° C. to 800° C. in air at aconstant stress of 100 psi. Cycle sscm (total) sscm/cm 1 0.541 0.0086 10.545 0.0087 1 0.558 0.0089 2 0.681 0.0108 2 0.737 0.0117 2 0.733 0.01163 0.849 0.0135 3 0.853 0.0135 3 0.857 0.0136 3 0.827 0.0131 4 0.8140.0129 4 0.813 0.0129 4 0.816 0.0130 8 1.14 0.0181 8 1.14 0.0181 8 1.140.0181

[0108] It is evident that the multi-layer seal showed very goodstability under thermal cycling. Optical microscopy also confirmed thatno melting of mica was observed and the individual layers (SS430 andmica layers) were easily separated after the test.

[0109] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only selected embodiments have been shown and described and thatall changes, equivalents, and modifications that come within the spiritof the invention described herein or defined by the following claims aredesired to be protected. Any experiments, experimental examples, orexperimental results provided herein are intended to be illustrative ofthe present invention and should not be considered limiting orrestrictive with regard to the invention scope. Further, any theory,mechanism of operation, or finding stated herein is meant to furtherenhance understanding of the present invention and is not intended tolimit the present invention in any way to such theory, mechanism orfinding. All publications, patents, and patent applications cited inthis specification are herein incorporated by reference as if eachindividual publication, patent, or patent application were specificallyand individually indicated to be incorporated by reference and set forthin its entirety herein.

What is claimed is:
 1. An electrochemical device, comprising: aplurality of components, said components forming at least one boundarybetween diverse gaseous streams and defining at least one junctiontherebetween; a multi-layer seal positioned at the junction, the sealcomposed of a gasket body disposed between two compliant interlayers,wherein each compliant interlayer is positioned between the gasket bodyand one of said components; and a compression member for exerting acompressive force to the components and the seal.
 2. The electrochemicaldevice in accordance with claim 1 wherein said seal is a non-conductingseal.
 3. The electrochemical device in accordance with claim 1 whereinthe gasket body comprises mica.
 4. The electrochemical device inaccordance with claim 1 wherein at least one of said compliantinterlayers comprises glass.
 5. The electrochemical device in accordancewith claim 4 wherein the glass has a softening point lower than or equalto the operating temperatures of the device.
 6. The electrochemicaldevice in accordance with claim 4 wherein the glass is not corrosive tosurfaces of the components in contact with the glass under operatingconditions.
 7. The electrochemical device in accordance with claim 4wherein the glass includes an alkaline earth element selected from thegroup consisting of strontium, magnesium and calcium or an alkaliadditive selected from the group consisting of sodium, potassium andlithium.
 8. The electrochemical device in accordance with claim 4wherein the glass comprises a borosilicate glass.
 9. The electrochemicaldevice in accordance with claim 4 wherein the glass interlayer has athickness of from about 0.005 millimeters to about 5 millimeters priorto heating.
 10. The electrochemical device in accordance with claim 1wherein at least one of said compliant layers comprises a metal.
 11. Theelectrochemical device in accordance with claim 10 wherein the metal isresistant to oxidation under operating conditions of the device.
 12. Theelectrochemical device in accordance with claim 10 wherein the metal isselected from the group consisting of gold, silver, palladium andplatinum.
 13. The electrochemical device in accordance with claim 10wherein the compliant interlayer is a metallic foil having a thicknessof from about 0.005 millimeters to about 1 millimeters prior to heating.14. The electrochemical device in accordance with claim 1 wherein thecompressive force is a force of from about 5 to about 500 psi.
 15. Theelectrochemical device in accordance with claim 1 wherein said gasketbody has a thickness of from about 25 microns to about 2 millimeters.16. The electrochemical device in accordance with claim 1 wherein eachof said compliant layers has a thickness of from about 0.005 millimetersto about 1 millimeter prior to heating.
 17. The electrochemical devicein accordance with claim 1 wherein the electrochemical device comprisesa member selected from the group consisting of a solid oxide fuel cell,a syngas membrane reactor and an oxygen generator.
 18. A multi-layerseal for sealing a junction between adjacent components of anelectrochemical device, said seal comprising: a gasket body definingfirst and second opposing surfaces; a first compliant interlayerpositioned adjacent the first surface; and a second compliant interlayerpositioned adjacent the second surface.
 19. The seal in accordance withclaim 18 wherein the opposing surfaces of said gasket body areconfigured to correspond to junction surfaces of the adjacentcomponents.
 20. The seal in accordance with claim 19 wherein each ofsaid first and second compliant interlayers is positioned to be disposedbetween said gasket body and the junction surface of one of the adjacentcomponents.
 21. The seal in accordance with claim 18 wherein the gasketbody comprises a member selected from the group consisting of a singlecrystal mica and a mica paper.
 22. The seal in accordance with claim 18wherein said gasket body comprises a mica selected from the groupconsisting of Muscovite, Phlogopite, Biotite, Fuchsite, Lepidolite andZinnwaldite.
 23. The seal in accordance with claim 18 wherein at leastone of said first and second compliant interlayers comprises a memberselected from the group consisting of a glass, a glass-ceramic, a micaglass-ceramic, a phase-separated glass, a glass composite, a cermet, ametal, a metal alloy and a metal composite.
 24. The seal in accordancewith claim 18 wherein at least one of said first and second compliantinterlayers comprises glass.
 25. The seal in accordance with claim 24wherein the glass has a softening point lower than or equal to theoperating temperature of the electrochemical device.
 26. The seal inaccordance with claim 24 wherein the glass includes an alkaline earthelement selected from the group consisting of strontium, magnesium andcalcium or an alkali additive selected from the group consisting ofsodium, potassium and lithium.
 27. The seal in accordance with claim 24wherein the glass comprises a borosilicate glass.
 28. The seal inaccordance with claim 18 wherein at least one of said first and secondcompliant interlayers comprises a metal.
 29. The seal in accordance withclaim 28 wherein the metal is resistant to oxidation under operatingconditions of the electrochemical device.
 30. The seal in accordancewith claim 28 wherein the metal is selected from the group consisting ofgold, silver, palladium and platinum.
 31. The seal in accordance withclaim 28 wherein at least one of said first and second compliantinterlayers is a metallic foil having a thickness of from about 0.005millimeters to about 1 millimeter.
 32. The seal in accordance with claim18 wherein said gasket body has a thickness of from about 25 microns toabout 1 millimeter.
 33. The seal in accordance with claim 18 whereineach of said compliant layers has a thickness of from about 0.005millimeters to about 1 millimeter.
 34. A method for making a multi-layerseal, comprising: providing a gasket body defining first and secondgenerally flat opposing surfaces; and applying a compliant material tosaid first and second surfaces to form first and second compliantinterlayers.
 35. The method in accordance with claim 34 wherein thegasket body comprises mica.
 36. The method in accordance with claim 34wherein at least one of the first and second compliant interlayerscomprises a member selected from the group consisting of a glass, aglass-ceramic, a mica glass-ceramic, a phase-separated glass, a glasscomposite, a cermet, a metal, a metal alloy and a metal composite. 37.The method in accordance with claim 34 wherein the gasket body has athickness of from about 25 microns to about 1 millimeter.
 38. The methodin accordance with claim 34 wherein each of the first and secondcompliant layers has a thickness of from about 0.005 millimeters toabout 1 millimeter.
 39. The method in accordance with claim 34 whereinsaid applying comprises a member selected from the group consisting ofdip-coating, painting, screen printing, deposition, spattering, tapecasting and sedimentation.
 40. A method for sealing a junction betweenadjacent ceramic or metallic components of an electrochemical device,comprising: positioning between the adjacent components a multi-layerseal composed of a gasket body disposed between a first compliantinterlayer and a second compliant interlayer, wherein each of the firstand second compliant interlayers is positioned between the gasket bodyand one of the components; and applying a compressive force to thecomponents and the seal.
 41. The method in accordance with claim 40wherein the gasket body comprises a member selected from the groupconsisting of a single crystal mica and a mica paper.
 42. The method inaccordance with claim 40 wherein at least one of the first and secondcompliant interlayers comprises a member selected from the groupconsisting of glass and metal.
 43. The method in accordance with claim40 wherein the compressive force is a force of from about 5 to about 500psi.
 44. The method in accordance with claim 40 wherein the gasket bodyhas a thickness of from about 25 microns to about 1 millimeter.
 45. Themethod in accordance with claim 40 wherein each of the first and secondcompliant layers has a thickness of from about 0.005 millimeters toabout 1 millimeter.
 46. A solid oxide fuel cell assembly forelectrochemically reacting a fuel gas with a flowing oxidant gas at anelevated temperature to produce a DC output voltage, said solid oxidefuel cell comprising: a plurality of generally planar integral fuel cellunits, each unit comprising a layer of ceramic ion conductingelectrolyte disposed between a conductive anode layer and a conductivecathode layer, wherein said units are arranged one on another along alongitudinal axis perpendicular to said planar units to form a fuel cellstack; a multi-layer non-conducting seal disposed between the anodelayer and the cathode layer of adjacent fuel cell units, wherein theseal is composed of a gasket body disposed between two compliantinterlayers; and a compression member for exerting a compressive forcealong the longitudinal axis.
 47. The assembly in accordance with claim46 wherein the compressive force is a force of from about 5 to about 500psi.
 48. The assembly in accordance with claim 46 wherein the anodelayer is composed of a first porous ceramic material and the cathodelayer is composed of a second porous ceramic material.